1. Technical Field of the Invention
The present invention relates in general to the telecommunications field and, in particular, to a method for estimating downlink interference in a cellular communications system using a Broadcast Control Channel Allocation (BA) list to determine cell-to-cell interdependencies.
2. Description of Related Art
In order to improve the radio environment of today's cellular communications systems, many operators utilize sector cell (uni-directional) antenna systems, and automated planning techniques to assist with network planning and cell assignment decisions. One such automated planning technique is referred to as (slow) Adaptive Frequency Allocation (AFA), whereby a network operator utilizes an iterative algorithm to automatically and frequently reconfigure the network's cell plan (to minimize radio interference) and gradually improve the quality of the radio environment.
Nevertheless, although AFA is recognized as a substantial improvement over prior network planning approaches, its use creates a significant planning and cell assignment problem. For example, AFA algorithms are currently designed to make cell assignment decisions based on measurements of uplink radio interference in the cells. However, when an operator utilizes AFA in a network with a sector cell antenna system, the uplink and downlink interference measurements made in any given cell (i.e., for a specific channel at a specific point in time) can be poorly correlated. In other words, for any given cell in the network, the uplink measurements that are made do not record the radio interference created by mobile terminals located in the area behind that (sector) cell's base transceiver station (BTS) antenna. However, radio transmissions from other cells located in that same area will create interference on the downlink of the given cell. An example that illustrates the poor correlation between such uplink and downlink measurements is shown in the cell plan of FIG. 1.
Referring to FIG. 1, if an interference measurement were to be made on the uplink in cell A, then interference from cells A120, A180, and A240 would be difficult to detect. Consequently, for that situation, an AFA algorithm might "suggest" to the network operator that cell A use the same frequencies as those used in cells A120, A180 and A240. However, implementation of this "suggestion" would create a significant problem for the mobile terminals located in cell A, because radio transmissions originating in cells A120, A180 and A240 would create interference in cell A on the downlink. Notably, this problem could be avoided, if the downlink interference being created in cell A could be adequately measured or at least accurately estimated.
Numerous problems are encountered when attempts are made to measure downlink radio interference for frequency planning purposes. For example, one technique that can be used is to measure the downlink interference right at the network's base station. However, since the downlink interference is being measured at only one point (e.g., where the base station receiver's antenna is located), this lone reading is inadequate from a testing and operational standpoint. A better approach would be to measure the downlink interference at each of the mobile stations' locations in the cell. The mobile stations would make the measurements and report them to the base station.
Another technique that can be used for downlink interference measurements is to place the measurement equipment at a number of different, fixed locations in a cell. Consequently, the downlink interference in the cell can be measured at all of those fixed locations. Unfortunately, this approach requires a network operator to purchase a substantial amount of additional measurement equipment, which is quite costly to install and maintain. In fact, this added cost typically outweighs the benefits that can be derived from making downlink radio interference measurements at a number of fixed locations in a cell. In other words, in order to obtain the best results with such a downlink measurement approach, the measurement equipment should be located where the majority of the cell traffic occurs (assuming optimistically that the network operator will always have such knowledge beforehand).
Still another technique that can be used for measuring downlink interference in a cellular network is to use measurements based on the mobile's BA lists (or a similar list of frequencies). For example, in the cellular Global System for Mobile Communications (GSM), the GSM mobile terminals measure downlink signal strength only on BCCH frequencies. These BCCH frequencies are defined by the network operator and placed on the mobile's BA lists. However, this technique is limited because the mobile terminals' downlink signal strength measurements are confined to those BCCH frequencies on the BA list, which is no more than a list of the BCCH frequencies used in "neighboring" cells (i.e., cells bordering on one another). Furthermore, of those BCCH frequencies on the BA lists, each GSM mobile terminal can report back to the network's base station only those measurements made for the six strongest BCCH frequencies, where the terminal has successfully decoded the associated Base Station Identity Codes (BSICs).